The present application relates to diode and rectifier structures and methods, and more particularly to structures and methods which include positive feedback in the device-level operation.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
Diodes are very common circuit elements used to perform rectification of an oscillating input signal. A typical rectifying diode has two external electrodes, and conducts current readily in a forward direction (ON state), and permits only small or no current flows in the reverse direction (OFF state). Schottky barrier diodes and synchronous rectifiers are typically used to perform this function for signals with amplitudes of less than 10V.
Schottky barrier diodes are simple to use but not very efficient. Typically they have a forward voltage drop above 0.35V at room temperature. If the output signal is 3.3V, then approximately 10% of the power will be wasted for rectification. This large energy loss for rectification is unacceptable for modern power supplies.
The ideal diode equation limits the forward voltage drop VF for a given rectification ratio to
      V    F    >            kT      q        ⁢          ln      ⁡              (                  1          +                                    I              F                        /                          I              R                                      )            where IF is a forward current, VF is the forward bias voltage, IR is the leakage current, k is Boltzmann's constant, and kT/q=0.0259V at room temperature T.
For example, for a Schottky diode conducting a current of 2 A, with 20 μA leakage current, the forward voltage drop is larger than 0.3V. There is very small room for improvement of Schottky diodes since they cannot be better than ideal.
To overcome the high losses on rectifiers for low voltage applications, synchronous rectification is often used. (See e.g. Cryssis G., “High Frequency Switching Power Supplies: Theory and Design”, McGraw-Hill, Inc., 2 edition 1989, p. 144. This entire book is hereby incorporated by reference.) Use of a MOSFET to perform the rectification function of the diode allows voltage drop on a rectifier be reduced to about 0.1V, leading to increased efficiency. However, the circuit implementation of synchronous rectification becomes more complicated. A controller is needed to provide the gate voltage and to change the MOSFET from the ON to the OFF state. Sensors are needed to tell the controller that the sign of the applied voltage has changed. This additional signal processing reduces both the speed of operation and reliability, and also substantially increases the cost of synchronous rectifiers because instead of a simple diode, one needs a much more complicated and expensive circuit.
Prior applications of the present inventors have described a “Regenerative Building Block” device, or “RBB.” This is a four-terminal device, which not only includes source/gate/drift/drain operating conventionally, but also a probe node which is connected to the drift region separately from the drain. As shown in Published US application US2009/0185404, the probe node can for example be positioned as if it were a lateral DMOS drain, while the main current flow goes vertically downward to a backside drain contact. Two such RBBs can be connected to provide a half-bridge, and two such half-bridges of opposite polarity can be connected together to provide a full-wave rectifier. The RBB itself is a useful building block, which is also used in some embodiments of the present application.